G2A usually gets the keys from illegal places, so the dev doesn't make a single penny when you buy a key off G2A. Then, if Steam finds out that the keys are fake (which it often does) the people who bought the keys get a refund. Which the developer has to pay for. So not only does the dev not make any cash, they lose money if the keys are fake and get detected by Steam. Whereas if you just pirate shit, the dev doesn't gain anything, but they don't lose anything either.

There's been, like, 20 fucking scandals about this shit and g2a. They are massive fucking scumbags that fuck over everyone.